1. Field of the Invention
The present invention relates to print heads for electrostatic printing systems.
2. Discussion of the Prior Art
FIG. 7 of U.S. Pat. No. 3,689,935, entitled "Electrostatic Line Printer," discloses an electrostatic particle modulator 50 having two staggered rows of modulated apertures for forming dot-matrix images on a moving print medium. The individual circular apertures are surrounded by individual electrodes which permit modulation of toner particles propelled through the apertures. Rather than projecting toner particles directly through the modulated apertures, the particle modulator can be used to modulate a stream of ions to form latent images on a dielectric print medium which is then subjected to developing and fixing operations. The disclosed particles modulation system is, however, characterized by certain inherent limitations that tend to limit its utility in making high resolution prints such as bar codes, which are useful for encoding commercial product identification data on labels and the like.
It is believed that the difficulty in imaging high resolution information with the print head of U.S. Pat. No. 3,689,935 is caused by several factors. To print with the apparatus of FIG. 7 of U.S. Pat. No. 3,689,935, a first row of dots is imaged on the dielectric print medium from one or more of the apertures within either aperture row 53 or 54. At a later time which is a function of the velocity of the print medium, a second group of dots is imaged thereon from the remaining aperture row. After the first row of ion dots is imaged on the print medium, its charges tend to repel the like charges of the second row of dots as the latter are being imaged upon the print medium to fill in the vacant spaces left between the dots of the first row. The repulsion of the second row of dots by the first row of dots introduces a stagger into the merged row of dots and thus degrades the smoothness of printed edges that are perpendicular to the direction of motion of the dielectric print medium. A second factor is that the circular dot shapes produced by the circular apertures do not readily merge into straight-edged printed lines. A third factor arises out of the fact that the cross-sectional density of the circular ion beam defined by each aperture limits the density of the ion image deposited on the moving print medium.
Modification of the size and shape of electrostatically imaged dots in charge retentive media has been accomplished in the prior art, such as for example in U.S. Pat. No. 3,438,053, wherein it is suggested that the spot images deposited by a row of closely spaced pin-type electrodes may be rendered elliptical by the interposition of a common slot-shaped aperture between the pin electrodes and the print medium. The prior art does not, however, address the above-mentioned problems which are encountered when imaging is carried out by ion beam techniques on a moving medium using plural staggered rows of electrically modulated apertures as taught in the aforementioned U.S. Pat. No. 3,689,935.